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<h1 class="settitle" align="center">The C Preprocessor</h1>












<a name="SEC_Contents"></a>
<h2 class="contents-heading">Table of Contents</h2>

<div class="contents">

<ul class="no-bullet">
  <li><a name="toc-Overview-1" href="Overview.html#Overview">1 Overview</a>
  <ul class="no-bullet">
    <li><a name="toc-Character-sets-1" href="Character-sets.html#Character-sets">1.1 Character sets</a></li>
    <li><a name="toc-Initial-processing-1" href="Initial-processing.html#Initial-processing">1.2 Initial processing</a></li>
    <li><a name="toc-Tokenization-1" href="Tokenization.html#Tokenization">1.3 Tokenization</a></li>
    <li><a name="toc-The-preprocessing-language-1" href="The-preprocessing-language.html#The-preprocessing-language">1.4 The preprocessing language</a></li>
  </ul></li>
  <li><a name="toc-Header-Files-1" href="Header-Files.html#Header-Files">2 Header Files</a>
  <ul class="no-bullet">
    <li><a name="toc-Include-Syntax-1" href="Include-Syntax.html#Include-Syntax">2.1 Include Syntax</a></li>
    <li><a name="toc-Include-Operation-1" href="Include-Operation.html#Include-Operation">2.2 Include Operation</a></li>
    <li><a name="toc-Search-Path-1" href="Search-Path.html#Search-Path">2.3 Search Path</a></li>
    <li><a name="toc-Once_002dOnly-Headers-1" href="Once_002dOnly-Headers.html#Once_002dOnly-Headers">2.4 Once-Only Headers</a></li>
    <li><a name="toc-Alternatives-to-Wrapper-_0023ifndef-1" href="Alternatives-to-Wrapper-_0023ifndef.html#Alternatives-to-Wrapper-_0023ifndef">2.5 Alternatives to Wrapper #ifndef</a></li>
    <li><a name="toc-Computed-Includes-1" href="Computed-Includes.html#Computed-Includes">2.6 Computed Includes</a></li>
    <li><a name="toc-Wrapper-Headers-1" href="Wrapper-Headers.html#Wrapper-Headers">2.7 Wrapper Headers</a></li>
    <li><a name="toc-System-Headers-1" href="System-Headers.html#System-Headers">2.8 System Headers</a></li>
  </ul></li>
  <li><a name="toc-Macros-1" href="Macros.html#Macros">3 Macros</a>
  <ul class="no-bullet">
    <li><a name="toc-Object_002dlike-Macros-1" href="Object_002dlike-Macros.html#Object_002dlike-Macros">3.1 Object-like Macros</a></li>
    <li><a name="toc-Function_002dlike-Macros-1" href="Function_002dlike-Macros.html#Function_002dlike-Macros">3.2 Function-like Macros</a></li>
    <li><a name="toc-Macro-Arguments-1" href="Macro-Arguments.html#Macro-Arguments">3.3 Macro Arguments</a></li>
    <li><a name="toc-Stringification-1" href="Stringification.html#Stringification">3.4 Stringification</a></li>
    <li><a name="toc-Concatenation-1" href="Concatenation.html#Concatenation">3.5 Concatenation</a></li>
    <li><a name="toc-Variadic-Macros-1" href="Variadic-Macros.html#Variadic-Macros">3.6 Variadic Macros</a></li>
    <li><a name="toc-Predefined-Macros-1" href="Predefined-Macros.html#Predefined-Macros">3.7 Predefined Macros</a>
    <ul class="no-bullet">
      <li><a name="toc-Standard-Predefined-Macros-1" href="Standard-Predefined-Macros.html#Standard-Predefined-Macros">3.7.1 Standard Predefined Macros</a></li>
      <li><a name="toc-Common-Predefined-Macros-1" href="Common-Predefined-Macros.html#Common-Predefined-Macros">3.7.2 Common Predefined Macros</a></li>
      <li><a name="toc-System_002dspecific-Predefined-Macros-1" href="System_002dspecific-Predefined-Macros.html#System_002dspecific-Predefined-Macros">3.7.3 System-specific Predefined Macros</a></li>
      <li><a name="toc-C_002b_002b-Named-Operators-1" href="C_002b_002b-Named-Operators.html#C_002b_002b-Named-Operators">3.7.4 C++ Named Operators</a></li>
    </ul></li>
    <li><a name="toc-Undefining-and-Redefining-Macros-1" href="Undefining-and-Redefining-Macros.html#Undefining-and-Redefining-Macros">3.8 Undefining and Redefining Macros</a></li>
    <li><a name="toc-Directives-Within-Macro-Arguments-1" href="Directives-Within-Macro-Arguments.html#Directives-Within-Macro-Arguments">3.9 Directives Within Macro Arguments</a></li>
    <li><a name="toc-Macro-Pitfalls-1" href="Macro-Pitfalls.html#Macro-Pitfalls">3.10 Macro Pitfalls</a>
    <ul class="no-bullet">
      <li><a name="toc-Misnesting-1" href="Misnesting.html#Misnesting">3.10.1 Misnesting</a></li>
      <li><a name="toc-Operator-Precedence-Problems-1" href="Operator-Precedence-Problems.html#Operator-Precedence-Problems">3.10.2 Operator Precedence Problems</a></li>
      <li><a name="toc-Swallowing-the-Semicolon-1" href="Swallowing-the-Semicolon.html#Swallowing-the-Semicolon">3.10.3 Swallowing the Semicolon</a></li>
      <li><a name="toc-Duplication-of-Side-Effects-1" href="Duplication-of-Side-Effects.html#Duplication-of-Side-Effects">3.10.4 Duplication of Side Effects</a></li>
      <li><a name="toc-Self_002dReferential-Macros-1" href="Self_002dReferential-Macros.html#Self_002dReferential-Macros">3.10.5 Self-Referential Macros</a></li>
      <li><a name="toc-Argument-Prescan-1" href="Argument-Prescan.html#Argument-Prescan">3.10.6 Argument Prescan</a></li>
      <li><a name="toc-Newlines-in-Arguments-1" href="Newlines-in-Arguments.html#Newlines-in-Arguments">3.10.7 Newlines in Arguments</a></li>
    </ul></li>
  </ul></li>
  <li><a name="toc-Conditionals-1" href="Conditionals.html#Conditionals">4 Conditionals</a>
  <ul class="no-bullet">
    <li><a name="toc-Conditional-Uses-1" href="Conditional-Uses.html#Conditional-Uses">4.1 Conditional Uses</a></li>
    <li><a name="toc-Conditional-Syntax-1" href="Conditional-Syntax.html#Conditional-Syntax">4.2 Conditional Syntax</a>
    <ul class="no-bullet">
      <li><a name="toc-Ifdef-1" href="Ifdef.html#Ifdef">4.2.1 Ifdef</a></li>
      <li><a name="toc-If-1" href="If.html#If">4.2.2 If</a></li>
      <li><a name="toc-Defined-1" href="Defined.html#Defined">4.2.3 Defined</a></li>
      <li><a name="toc-Else-1" href="Else.html#Else">4.2.4 Else</a></li>
      <li><a name="toc-Elif-1" href="Elif.html#Elif">4.2.5 Elif</a></li>
    </ul></li>
    <li><a name="toc-Deleted-Code-1" href="Deleted-Code.html#Deleted-Code">4.3 Deleted Code</a></li>
  </ul></li>
  <li><a name="toc-Diagnostics-1" href="Diagnostics.html#Diagnostics">5 Diagnostics</a></li>
  <li><a name="toc-Line-Control-1" href="Line-Control.html#Line-Control">6 Line Control</a></li>
  <li><a name="toc-Pragmas-1" href="Pragmas.html#Pragmas">7 Pragmas</a></li>
  <li><a name="toc-Other-Directives-1" href="Other-Directives.html#Other-Directives">8 Other Directives</a></li>
  <li><a name="toc-Preprocessor-Output-1" href="Preprocessor-Output.html#Preprocessor-Output">9 Preprocessor Output</a></li>
  <li><a name="toc-Traditional-Mode-1" href="Traditional-Mode.html#Traditional-Mode">10 Traditional Mode</a>
  <ul class="no-bullet">
    <li><a name="toc-Traditional-lexical-analysis-1" href="Traditional-lexical-analysis.html#Traditional-lexical-analysis">10.1 Traditional lexical analysis</a></li>
    <li><a name="toc-Traditional-macros-1" href="Traditional-macros.html#Traditional-macros">10.2 Traditional macros</a></li>
    <li><a name="toc-Traditional-miscellany-1" href="Traditional-miscellany.html#Traditional-miscellany">10.3 Traditional miscellany</a></li>
    <li><a name="toc-Traditional-warnings-1" href="Traditional-warnings.html#Traditional-warnings">10.4 Traditional warnings</a></li>
  </ul></li>
  <li><a name="toc-Implementation-Details-1" href="Implementation-Details.html#Implementation-Details">11 Implementation Details</a>
  <ul class="no-bullet">
    <li><a name="toc-Implementation_002ddefined-behavior-1" href="Implementation_002ddefined-behavior.html#Implementation_002ddefined-behavior">11.1 Implementation-defined behavior</a></li>
    <li><a name="toc-Implementation-limits-1" href="Implementation-limits.html#Implementation-limits">11.2 Implementation limits</a></li>
    <li><a name="toc-Obsolete-Features-1" href="Obsolete-Features.html#Obsolete-Features">11.3 Obsolete Features</a>
    <ul class="no-bullet">
      <li><a name="toc-Assertions" href="Obsolete-Features.html#Assertions">11.3.1 Assertions</a></li>
    </ul></li>
    <li><a name="toc-Differences-from-previous-versions-1" href="Differences-from-previous-versions.html#Differences-from-previous-versions">11.4 Differences from previous versions</a></li>
  </ul></li>
  <li><a name="toc-Invocation-1" href="Invocation.html#Invocation">12 Invocation</a></li>
  <li><a name="toc-Environment-Variables-1" href="Environment-Variables.html#Environment-Variables">13 Environment Variables</a></li>
  <li><a name="toc-GNU-Free-Documentation-License-1" href="GNU-Free-Documentation-License.html#GNU-Free-Documentation-License">GNU Free Documentation License</a>
  <ul class="no-bullet">
    <li><a name="toc-ADDENDUM_003a-How-to-use-this-License-for-your-documents" href="GNU-Free-Documentation-License.html#ADDENDUM_003a-How-to-use-this-License-for-your-documents">ADDENDUM: How to use this License for your documents</a></li>
  </ul></li>
  <li><a name="toc-Index-of-Directives-1" href="Index-of-Directives.html#Index-of-Directives">Index of Directives</a></li>
  <li><a name="toc-Option-Index-1" href="Option-Index.html#Option-Index">Option Index</a></li>
  <li><a name="toc-Concept-Index-1" href="Concept-Index.html#Concept-Index">Concept Index</a></li>
</ul>
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Next: <a href="Overview.html#Overview" accesskey="n" rel="next">Overview</a>, Up: <a href="../dir/index.html" accesskey="u" rel="up">(dir)</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p>
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<p>The C preprocessor implements the macro language used to transform C,
C++, and Objective-C programs before they are compiled.  It can also be
useful on its own.
</p>
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<tr><td align="left" valign="top">&bull; <a href="Traditional-Mode.html#Traditional-Mode">Traditional Mode</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
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<tr><td align="left" valign="top">&bull; <a href="Concept-Index.html#Concept-Index">Concept Index</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><th colspan="3" align="left" valign="top"><pre class="menu-comment">
</pre></th></tr><tr><th colspan="3" align="left" valign="top"><pre class="menu-comment"> &mdash; The Detailed Node Listing &mdash;

Overview

</pre></th></tr><tr><td align="left" valign="top">&bull; <a href="Character-sets.html#Character-sets">Character sets</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Initial-processing.html#Initial-processing">Initial processing</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Tokenization.html#Tokenization">Tokenization</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="The-preprocessing-language.html#The-preprocessing-language">The preprocessing language</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><th colspan="3" align="left" valign="top"><pre class="menu-comment">
Header Files

</pre></th></tr><tr><td align="left" valign="top">&bull; <a href="Include-Syntax.html#Include-Syntax">Include Syntax</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Include-Operation.html#Include-Operation">Include Operation</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Search-Path.html#Search-Path">Search Path</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Once_002dOnly-Headers.html#Once_002dOnly-Headers">Once-Only Headers</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Alternatives-to-Wrapper-_0023ifndef.html#Alternatives-to-Wrapper-_0023ifndef">Alternatives to Wrapper #ifndef</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
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</td></tr>
<tr><td align="left" valign="top">&bull; <a href="System-Headers.html#System-Headers">System Headers</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><th colspan="3" align="left" valign="top"><pre class="menu-comment">
Macros

</pre></th></tr><tr><td align="left" valign="top">&bull; <a href="Object_002dlike-Macros.html#Object_002dlike-Macros">Object-like Macros</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Function_002dlike-Macros.html#Function_002dlike-Macros">Function-like Macros</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Macro-Arguments.html#Macro-Arguments">Macro Arguments</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
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</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Concatenation.html#Concatenation">Concatenation</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Variadic-Macros.html#Variadic-Macros">Variadic Macros</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Predefined-Macros.html#Predefined-Macros">Predefined Macros</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Undefining-and-Redefining-Macros.html#Undefining-and-Redefining-Macros">Undefining and Redefining Macros</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Directives-Within-Macro-Arguments.html#Directives-Within-Macro-Arguments">Directives Within Macro Arguments</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Macro-Pitfalls.html#Macro-Pitfalls">Macro Pitfalls</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><th colspan="3" align="left" valign="top"><pre class="menu-comment">
Predefined Macros

</pre></th></tr><tr><td align="left" valign="top">&bull; <a href="Standard-Predefined-Macros.html#Standard-Predefined-Macros">Standard Predefined Macros</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Common-Predefined-Macros.html#Common-Predefined-Macros">Common Predefined Macros</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="System_002dspecific-Predefined-Macros.html#System_002dspecific-Predefined-Macros">System-specific Predefined Macros</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="C_002b_002b-Named-Operators.html#C_002b_002b-Named-Operators">C++ Named Operators</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><th colspan="3" align="left" valign="top"><pre class="menu-comment">
Macro Pitfalls

</pre></th></tr><tr><td align="left" valign="top">&bull; <a href="Misnesting.html#Misnesting">Misnesting</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Operator-Precedence-Problems.html#Operator-Precedence-Problems">Operator Precedence Problems</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Swallowing-the-Semicolon.html#Swallowing-the-Semicolon">Swallowing the Semicolon</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Duplication-of-Side-Effects.html#Duplication-of-Side-Effects">Duplication of Side Effects</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Self_002dReferential-Macros.html#Self_002dReferential-Macros">Self-Referential Macros</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Argument-Prescan.html#Argument-Prescan">Argument Prescan</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Newlines-in-Arguments.html#Newlines-in-Arguments">Newlines in Arguments</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><th colspan="3" align="left" valign="top"><pre class="menu-comment">
Conditionals

</pre></th></tr><tr><td align="left" valign="top">&bull; <a href="Conditional-Uses.html#Conditional-Uses">Conditional Uses</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
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<tr><td align="left" valign="top">&bull; <a href="Conditional-Syntax.html#Conditional-Syntax">Conditional Syntax</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Deleted-Code.html#Deleted-Code">Deleted Code</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><th colspan="3" align="left" valign="top"><pre class="menu-comment">
Conditional Syntax

</pre></th></tr><tr><td align="left" valign="top">&bull; <a href="Ifdef.html#Ifdef">Ifdef</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="If.html#If">If</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Defined.html#Defined">Defined</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Else.html#Else">Else</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Elif.html#Elif">Elif</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><th colspan="3" align="left" valign="top"><pre class="menu-comment">
Implementation Details

</pre></th></tr><tr><td align="left" valign="top">&bull; <a href="Implementation_002ddefined-behavior.html#Implementation_002ddefined-behavior">Implementation-defined behavior</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Implementation-limits.html#Implementation-limits">Implementation limits</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Obsolete-Features.html#Obsolete-Features">Obsolete Features</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Differences-from-previous-versions.html#Differences-from-previous-versions">Differences from previous versions</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><th colspan="3" align="left" valign="top"><pre class="menu-comment">
Obsolete Features

</pre></th></tr><tr><td align="left" valign="top">&bull; <a href="Obsolete-Features.html#Obsolete-Features">Obsolete Features</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><th colspan="3" align="left" valign="top"><pre class="menu-comment">
</pre></th></tr></table>

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<p>Permission is granted to copy, distribute and/or modify this document
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<p>This manual contains no Invariant Sections.  The Front-Cover Texts are
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